Agriculture Reference
In-Depth Information
converted into biomass, and (3) the proportion of growth allocated to
harvestable plant parts (
i.e.,
the harvest index). Of these three processes,
increasing the efficiency of radiation interception and increasing the
harvest index are the most promising avenues to increased yields. Cannell
et al.
(1987) showed that the efficiency of radiation interception is strongly
increased by early establishment of the crop canopy. Whereas there is little
scope for breeding for a more efficient use of the intercepted light,
increased partitioning to harvestable plant parts has been a successful
strategy in many crops. However, it should be noted that in biomass crops,
increasing partitioning to stems may come at the cost of enlarging growth
capacity through reduced partitioning to leaves.
Apart from canopy development and partitioning, other crop traits
that relate to the specific production system of the biomass crops are
analysed quantitatively. In
S. viminalis,
these traits determine crop duration
and cutting cycle length.
S. viminalis
is a perennial crop. After 20-30 years,
crop growth starts to decline and the crop is cleared. In the first years after
planting, crop growth is reduced compared to that of a full-grown crop.
Selecting for crop varieties that have a longer period of maximum growth
and therefore allow for a longer duration of the crop, minimize the effect of
reduced growth in the initial years.
S. viminalis
is harvested every 3-5
years. The time between two harvests is called the cutting cycle. In the first
year after harvest, growth is reduced compared to the later years of the
cutting cycle. Within a cutting cycle, growth starts to decline after 5 years
(Sennerby-Forsse
et al.
1992). Selecting for crop varieties that maintain a
high growth capacity throughout a longer cutting cycle may lead to a
longer period between two harvests, and thus to a higher annual biomass
increment, averaged over the entire cutting cycle.
In
M.
×
giganteus,
traits that determine crop duration and winter
losses are analysed.
M.
×
giganteus
is a perennial grass, which is harvested
every year. Crop growth starts to decline after 10-15 years. Also for
M
.×
giganteus,
the average yield may be increased by selecting for a longer
period of maximum growth. Usually,
M.
×
giganteus
is harvested at the
end of winter. Stem material may be lost as a result of reallocation to
rhizomes (underground plant parts) in the autumn, loss of stem tips owing
to winter storms, losses during harvest operations, and biomass remaining
in the stubble. Possible options to decrease these losses are breeding for
stronger stems,
e.g.
by inducing crop senescence earlier in the season, in
order to decrease lodging and breaking of stem tips, implementing
management that prohibits lodging, and improving harvest techniques in
order to minimize losses during harvest.
The statistical approach and the physiological approach are
complementary methods for estimating future crop yields. The statistical
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